Fix one cause of zombie buckd processes
Fix one cause of zombie buckd processes

Summary:
We've gotten reports of multiple `buckd` processes being created for
a single root, and `buck kill` doesn't make the extras go away. I've
been investigating the prevalence of this problem, and along the way
I found one cause of it. The comment in the code says the rest.

Reviewed By: styurin

fbshipit-source-id: 4a13247d7d

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